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Plastid terminal oxidase (PTOX) is a plastid-localized protein that acts as plastoquinol oxidase. The objective of this study was to determine the role of plastid terminal oxidase in response to drought stress and examine the differential response of PTOX pathway in both cultivated (Yousef and Morocco) and tolerant wild barley (spantaneum; HS) genotypes. Plants were subjected to different levels of soil water availability including control, mild and severe drought stress. Decrease in leaf relative water content as a result of progressive drought stressed led to decrease in stomatal conductance and increased enzymatic antioxidant machinery in all genotypes. Drought stress did not affect the PTOX transcript and protein level in cultivated genotypes Yousef and Morocco. However, PTOX gene expression and protein content significantly increased in HS genotype under severe drought. Assessment of PTOX activity via fast fluorescence induction curve in presence of PTOX specific inhibitor revealed that PTOX could efficiently oxidize plastoquinon pool and acts as alternative electron pathway in HS genotype. Results indicated that the role of PTOX in stress response was more significant in the wild barley than the cultivated barley varieties, where the emphasis has been on the desirable agronomic traits.

Heat shock proteins (HSPs) have vital roles during plant adaptation to biotic and abiotic stresses, as well as stress-free conditions. In the present study, we used a heterologous strategy of virus induced gene silencing to investigate the role of different classes of HSPs in Nicotiana benthamiana. Different growth and physiological parameters in silenced plants were evaluated under both control and salt stress conditions. Among the treatments, silencing of HSP70, especially under salinity regime, was found to have stronger impacts on growth, protein concentration, the accumulation of photosynthetic pigments, proline and total soluble carbohydrates content, malondialdehyde, the activity of antioxidant enzymes, performance index, relative water content and the ratio of K+/Na+, suggesting a more prominent role for HSP70 in both stressful and stress-free life cycle of the plants. Principal component analysis and hierarchical clustering indicated that HSP70 gene was silenced, the plants might effectively respond to stress by a higher accumulation of compatible solutes, like proline to protect the cell membranes and proteins from damage.

To study the effects of progressive drought stress on photosystem II behavior of wild type (Spantaneum) and cultivated barley (Morocco), different levels of soil water availability, including control, moderate, mild, and severe water stress (70%, 50%, 30%, and 10% water holding capacity of soil, respectively) and rehydration were used. Polyphasic OJIP fluorescence transient of Morocco plants exhibited a considerable increase in fluorescence intensity at O, J, and I steps under mild and severe stress relative to slight increase in wild barley. Values of fluorescence parameters and quantum efficiencies, including minimal fluorescence, relative variable fluorescence at phase J and I, maximal quantum yield of photosystem II (PSII), performance index, electron transport yield, and excitation transfer efficiency were influenced by water stress in both genotypes. These parameters were significantly less affected in wild type barley by progressive drought stress compared to Morocco. After re-watering, bot...

Effects of n-propyl gallate, a plastid terminal oxidase inhibitor involved in chlororespiration, on photosystem II efficiency in Dunaliella bardawil under low or high illumination was investigated. Rapid chlorophyll a fluorescence transients were recorded and analyzed according to JIP-test, which can quantify the photosystem II performance. The fluorescence transients O-J-I-P drastically decreased and almost reached a plateau when low light-grown cells were exposed for 96 h to 1, 2 and 4 mM n-propyl gallate. Very similar reductions in the efficiency of quantum yield of primary photochemistry (Φpo), the quantum yield for electron transport (ΦEo) and the inferred water-splitting complex activity (Fv/Fo) were found in the same inhibitor concentrations. However, no statistically significant change in fluorescence intensity and photosystem II efficiency was found when algal cells were exposed to the inhibitor concentrations up to 2 mM under high light intensity. The results indicated that inhibitory effects of n-propyl gallate on photosystem II electron flow in D. bardawil cells are dependent on environmental conditions. It is also demonstrated that n-propyl gallate is a multi-target inhibitor of growth kinetics as well as photosynthesis. In addition, we found that the donor side of photosystem II acts as main target place of the inhibitor.

In this work, we investigated the importance of phytoene desaturase (PDS) during salt stress in plants. For this aim, we transiently suppressed the corresponding gene of Nicotiana benthamiana using a Tobacco Rattle Virus silencing system. After salinity stress, the silenced plants were assayed for different growth and physiological parameters. The silenced plants were found to be induced more stunted and feeble shoots and roots than the negative controls. The results indicated that the concurrent incidence of salinity stress and viral infection did not have an additive influences on the plant growth and physiological parameters. Importantly, our results showed a more detrimental influence of salinity on plant growth and physiology after PDS silencing. Compared to the control, salt-stressed plants contained more proline, and malondialdehyde, less amount of photosynthetic pigments, higher Na+/K+ ratios in shoots and roots and lower relative water content after PDS silencing. The activity of antioxidant enzymes were also decreased in the plants coped with salinity or Tobacco Rattle Virus. It was also represented that simultaneous silencing of the PDS gene and salinity treatment resulted in a significant decrease of the performance index (PIABS) and an increase of dissipation per active reaction center (DIo/RC) reflecting severe injuries in photosynthetic machinery under such a condition. Overall results suggested that PDS silencing could increase the sensitivity of plants to salinity.

Drought stress is one of the most important abiotic factors which adversely affect growth, metabolism and yield of crops worldwide. The objective of this study was to determine the effects of drought stress on photosynthesis in barley and examine the differential responses of photosynthetic apparatus in relatively tolerant (Yousof) and susceptible (Morocco) barley genotypes. Plants were subjected to different levels of soil water availability including control, mild and severe drought stress. In both genotypes, drought stress led to decrease in chlorophylls, β-carotene and stomatal conductance accompanied by decrease in CO2 assimilation rate. Significant increase in α-tochoperol content was only observed in Yousof cultivar under drought stress. Initial slope and plateau phase of CO2 response curve of drought-stressed plants as well as polyphasic chlorophyll a fluorescence transient curve (OJIP test) and fast fluorescence induction kinetics were influenced by drought stress. These parameters were more affected in Morocco cultivar by drought stress compare with Yousof. Drought stress also resulted in reduction of D1 protein content in both genotypes and accelerated photoinhibition process. Based on our results, stomatal conductance is the main factor limiting photosynthesis in Yousof genotype under mild drought stress. However, in Morocco, in addition to stomatal limitation, damage to photosystem II (PSII), reduced electron transport and carboxylation efficiencies were important parts of limitation in photosynthesis. Severe drought stress resulted in structural and biochemical impairment of light-dependent reactions as well as carboxylation process of photosynthesis in both genotypes. Alpha-tocopherol showed an important protective role against drought stress in Yousof cultivar as a relatively drought-tolerant cultivar.

We studied the effects of n-propyl gallate, which is a plastid terminal oxidase inhibitor involved in chlororespiration, on photosynthesis and physiological parameters in Dunaliella salina grown under different salinities and under low or high irradiance using chlorophyll a fluorescence transient measurements and pigment analysis. The inhibitor up to 1 mM had an additive significant effect on the photosynthetic efficiency in the cell suspensions grown under low salinity and irradiance. However, in the presence of high n-propyl gallate concentration (4 mM), there was a negative effect on all physiological aspects. In contrast, this high concentration of the inhibitor could enhance efficiency of electron transport and growth parameters under high irradiance. On the other hand, with salinity increase, the unfavorable effects of high inhibitor concentration on the efficiency of photosystem II were less evident than of low salinity. Interestingly, n-propyl gallate high concentration had a positive effect on fluorescence and on physiological parameters when high salinities-grown cells were exposed to high irradiance. The results suggest that there is a rational correlation between increase of salinity and algae ability to bypass n-propyl gallate inhibited plastid terminal oxidase function and also direct influence of its lethal concentration on photosystem II compartment. The ability is especially substantial when the increase of salinity is accompanying high irradiance. Furthermore, these data show that algal responses to inhibitor concentrations are different under various environmental conditions.